Motion blur samples limit too low for a long exposure effect?
You could merge multiple renders and or use cycles.

But if the end effect you're going for is just abstract smeared lines, build them using curves and turn up bevel.  On the material, mix a transparent and a emission shader to create a similar effect.  and with eevee change the settings -> blend mode in the material to alpha blend.

This method will give you much more control over the effect too.

As a bonus: use mesh follow -> trace verts option in the Btrace addon to make curve traces from animated objects.
Here's an example I just made.